A suspected senior al-Qaeda operative is alleged to be among 12 terror suspects arrested in raids across England on Tuesday.
Suspects in Blackburn were dressed in forensic suits by armed police
He is thought to be a prominent figure in al-Qaeda, US officials told the BBC.
One of the 12, a 25-year-old man, was released without charge on Friday. The other men could be held by police until Sunday under anti-terror laws.
Home Secretary David Blunkett said police were "vigorously pursuing investigations" to protect the public.
He added: "We are maintaining a state of heightened readiness and taking every feasible precautionary measure to protect British citizens."
Custody limit extended
The 12 men are suspected of involvement in the commission, preparation or instigation of acts of terrorism.
Newsnight's diplomatic editor Mark Urban said speculation about the role of the man thought to be a senior al-Qaeda operative had "intensified...because a national security official has been briefing in the United States on this matter".
However, he said that often claims are made about an individual's importance in the organisation, despite the difficulty of proving those claims.
Meanwhile, police were given another 72 hours to question the 12 arrested men, bringing the custody time limit to Sunday evening.
The 12 are reported all to be of Asian origin but it is not yet known how many are British citizens.
They are being held at Paddington Green police station in London. A 13th man was freed without charge on Wednesday.
The men are also believed to have had contact with Mohammed Naeem Noor Khan, arrested in Pakistan on 12 July, Mark Urban said.
"We have established [Khan] was indeed a student at City University in central London in January 2003."
He enrolled in a course on human resource management and attended four of the ten lectures.
"It is definitely the case that some of those arrested in recent days in this country are under suspicion by the police because of contacts with him. That is part of the picture and we can definitely say that now," Mark Urban said.
Officers searched a number of "residential premises" in London, Bushey, Luton and Blackburn following the raids.
They say they are expected to take some time to complete.
In a statement on Tuesday, the Metropolitan Police said the raids were part of a "pre-planned, ongoing, intelligence-led operation".
"Today's operation is part of continuing and extensive inquiries by police and the security service into alleged international terrorism."
The raids came as Prime Minister Tony Blair faced pressure to spell out the level of the terrorism threat in the UK following a heightened alert in parts of the US.
The Home Office has said no specific threat has been uncovered, saying: "We are maintaining a state of heightened readiness in the UK."
According to Home Office figures, by May this year only 14 of the 562 people arrested under the Terrorism Act since 11 September 2001 had been convicted of charges under it.